Shape memory alloy (SMA) describes a variety of materials having the common property of contracting much like muscle fibers when heated, for example by application of an electrical current, and which expand to their original length upon cooling. Sheets, fibers, and wires comprised of SMA are therefore useful in numerous applications requiring physical movement in a small space with low to moderate cycling speeds. Advantageously, use of SMA in these applications eliminates or reduces the need for motors or solenoids, reducing weight, complexity, and cost of the resulting devices. For example, fine nickel-titanium (Ni—Ti) wires have been woven into cylindrical shapes for use as vascular stents. The stent is crushed and inserted into the desired position in a blood vessel. Upon warming to above its transformation temperature, the stent returns to the original cylindrical shape and reinforces the walls of the blood vessel.
Other applications for SMA include use as electrical connectors. At ambient temperatures, NiTi rings provide a secure joint for the connector. Chilling the rings causes them to release. In yet another unusual use for SMA, a NiTi spring in coffeepots is able to open a valve and release hot water at the proper temperature to brew a pot of coffee.
It is known also to fabricate various designs of actuators using SMA. Examples of these applications include one-directional actuators used for devices such as SMA cryogenic pipe fittings, bias force actuators, and differential SMA actuators (Liang, C. and Rogers, C. A., 1992. Design of Shape Memory Alloy Actuators. J. Mech. Design 114: 223, incorporated herein in its entirety by reference). Each device utilizes the physical properties of SMA, i.e. that under low temperature the material can be elongated to approximately 4-5% of its original length, and that it will contract that amount when heated. When cooled, if a sufficient biasing force is applied, the SMA will again elongate to approximately 4-5% of its original length.
The present invention provides a novel linear actuator utilizing SMA wire to provide the desired displacement force. The actuator of the present invention is adapted for single use, or may be configured in parallel or in serial orientation in accordance with the anticipated use. In one embodiment, a single actuator having one or more SMA wires may be used to generate a desired application of force. In another embodiment, a plurality of actuators may be configured in parallel and attached to a single plate, thereby providing a displacing force equal to the number of actuators used. In yet another embodiment, at least two actuators may be configured in serial orientation for use with a dispensing device, where it is desired to provide an automated incremental change in position of a dispensing device.